The Horror Writers Association can trace its origins back to author Robert R. McCammon, who worked with two friends, Joe R. Lansdale and his wife, Karen, to create the forerunner of the HWA. The original name of the organization was the Horror/Occult Writers League, or HOWL, which was eventually changed because most members thought it was keeping the organization from being taken seriously.
While the early organization included such noted authors as Dean Koontz, Robert Bloch, Joe, Karen and Robert, the heavy hitters, such as Stephen King, did not sign up. Eventually they did sign up, and the roster has included such noted names as Stephen King, Peter Straub and Clive Barker (also known as the Horror Holy Trinity to some). Even some folks whose work was geared towards other genres (but did include some horror elements), such as Ray Bradbury, joined after the organization was rolling.
The first meeting took place in 1985 at the World Fantasy Convention in Tucson, Arizona. The HWA is currently involved with the World Horror Convention, and alternate giving out the annual Stoker awards during the WHC or during a separate gathering, typically in Los Angeles or New York because of the publishing industry concentration in those areas.
The HWA published a newsletter starting in 1986, and continues to do so today. The current editor is author and Gila Queen, Kathryn Ptacek. Each issue notes what is currently happening in the industry, and includes interviews and essays for the genre.
The HWA has run a mentoring program off and on, depending on the needs of the organization. The newcomers can get paired with veterans to improve their craft. I was lucky enough to draw Joe Nassise as a mentor. They also run a grievance committee, and help to organize local chapters. I am currently working on this job as the Chapter Chairman.
Memberships are broken into several categories. Affiliate members have at least one professional sale, currently set at five cents per word. Active members have at least three pro sales over 7,500 words, or a novel to an established publisher. There are also non-writer memberships for publishers and agents. There are over 18 ways to join up or to advance to Active membership. The details are on the website (http://www.horror.org) under the Join link or the Membership Requirements link.
The HWA is a registered 501c non-profit organization, so all funds go towards advancing the genre. If you're interested in writing dark things, consider joining after your first pro sale.